setuid() sets the effective user ID of the calling process. If the
effective UID of the caller is root, the real UID and saved set-user-
ID are also set.
Under Linux, setuid() is implemented like the POSIX version with the
_POSIX_SAVED_IDS feature. This allows a set-user-ID (other than
root) program to drop all of its user privileges, do some un-
privileged work, and then reengage the original effective user ID in
a secure manner.
If the user is root or the program is set-user-ID-root, special care
must be taken. The setuid() function checks the effective user ID of
the caller and if it is the superuser, all process-related user ID's
are set to uid. After this has occurred, it is impossible for the
program to regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root
privileges, assume the identity of an unprivileged user, and then
regain root privileges afterward cannot use setuid(). You can
accomplish this with seteuid(2).

On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
Note: there are cases where setuid() can fail even when the caller is
UID 0; it is a grave security error to omit checking for a failure
return from setuid().

EAGAIN The call would change the caller's real UID (i.e., uid does
not match the caller's real UID), but there was a temporary
failure allocating the necessary kernel data structures.
EAGAIN uid does not match the real user ID of the caller and this
call would bring the number of processes belonging to the real
user ID uid over the caller's RLIMIT_NPROC resource limit.
Since Linux 3.1, this error case no longer occurs (but robust
applications should check for this error); see the description
of EAGAIN in execve(2).
EINVAL The user ID specified in uid is not valid in this user
namespace.
EPERM The user is not privileged (Linux: does not have the
CAP_SETUID capability) and uid does not match the real UID or
saved set-user-ID of the calling process.

Linux has the concept of the filesystem user ID, normally equal to
the effective user ID. The setuid() call also sets the filesystem
user ID of the calling process. See setfsuid(2).
If uid is different from the old effective UID, the process will be
forbidden from leaving core dumps.
The original Linux setuid() system call supported only 16-bit user
IDs. Subsequently, Linux 2.4 added setuid32() supporting 32-bit IDs.
The glibc setuid() wrapper function transparently deals with the
variation across kernel versions.

This page is part of release 3.81 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2014-09-21 SETUID(2)